The present invention relates to a system and method of drying chemical toner particles, and more particularly a system and method for drying chemical toner particles in a circulating flow of drying gas.
Chemical toner used in copiers is composed of toner particles which are applied to paper to produce an image. It is desirable that the toner particles be uniformly sized, having a narrow size distribution, to produce images with improved resolution and clarity. For example, in one known application, solid toner particles are produced having a typical average size distribution of approximately 6 microns in diameter with most particles falling in a range of about 2 to 8 microns.
It is also desirable that the toner particles flow freely during use to provide superior results. Moisture retained by the toner particles can cause the particles to stick together and not flow freely. During the process of manufacturing toner, the toner particles are dried until they have a moisture content sufficiently low enough that the toner particles do not stick together.
The toner particles are separated from each other in a process called deagglomeration. During drying, sufficient deagglomeration exposes the surface of each particle to enable efficient heat transfer from the particle which also aids in drying.
In a conventional process of forming chemical toner, latex particles and pigment particles are heated in a chemical reactor to form covalent bonds between the particles. The covalent bonds provide attractive forces between the particles causing them to come together or aggregate. The aggregated particles are then coalesced to make them more robust.
At this point in the process, the particles are in a liquid dispersion, also known as a mother liquor, which includes the toner particles, as well as residuals such as latex, pigment, surfactants, and other materials used in the process. Next, the mother liquor is dewatered from the particles to obtain a slurry including the solid toner particles as well as residuals including surfactants used to stabilize the latex, pigments and waxes. The wetcake is then washed to remove more of the residuals. The wetcake may be washed several times.
The washed toner particles, or wetcake, is then dried to provide free-flowing individual toner particles. Several different processes have been used for drying the toner particles, including indirect dryers such as disc dryers, drum dryers, paddle dryers, rotary dryers, and direct dryers including vacuum, freeze fluid bed and conveyers.
The wetcake includes a large number of different sized wet toner particles. Further, the moisture retained by each wet particle is typically proportional to the particle size, so that larger particles retain more moisture than smaller particles. A problem with conventional toner particle drying systems and methods, is that the drying time for each particle, that is the length of time each particle is dried, is approximately equal. Thus moisture retained by the particles is not removed in an effective and efficient manner. In order to remove enough of the moisture from the larger particles, conventional drying apparatus and processes often dry the smaller particles for too long, overheating them above their glass transition point (Tg).
Toner particles heated above their glass transition point (Tg) or melting point (Tm), can fuse with other particles. The fused toner particle clumps have sizes which exceed the desired range of particle size resulting in poor toner performance. It is desirable to dry each toner particle to remove the desired amount of moisture while preventing overheating which can result in the undesirable fusion of toner particles.
Additionally, the feed material enters the drying system as a de-lumped wetcake, and needs to be separated further to produce a free flowing product. Conventional toner drying technologies provide limited deagglomeration capabilities. It is desirable to improve deagglomeration during drying.
A system and method of drying wet toner particles used for hard-copy document and image reproduction.
In accordance with a first aspect of the invention a method of drying wet toner particles is provided. The method includes adding different sized wet toner particles to a dryer drying chamber and drying each of the wet toner particles for a drying time TD. The drying time TD can vary for each toner particle and is proportional to the mass of the wet toner particle.
In accordance with a second aspect of the invention, the method can include introducing a heated drying gas into the drying chamber to create a circulating flow of drying gas having a curved portion, and retaining the toner particles in the circulating flow until the toner particles lose enough moisture that exiting forces overcome centrifugal forces and the toner particles no longer remain in the drying chamber.
In accordance with another aspect of the invention, a dryer for drying toner particles is included. The dryer includes a drying chamber, at least one drying gas inlet for introducing heated drying gas into the drying chamber to produce a circulating flow of drying gas. The dryer includes a feed inlet for introducing wet toner particles into the circulating flow of drying gas to dry the wet toner particles. The dryer also includes an exit path communicating with the drying chamber for directing an exiting stream of the drying gas out of the drying chamber to move dry toner particles from the drying chamber.
Other features, benefits and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings.
The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps, preferred embodiments of which will be illustrated in the accompanying drawings wherein:
FIG. 1 is diagram illustrating a dryer in accordance with the invention;
FIG. 2 is diagram of a portion of the dryer show in FIG. 1 illustrating the forces exerted on a toner particle when the toner particle remains in the circulating stream in accordance with the invention;
FIG. 3 is diagram of a portion of the dryer show in FIG. 1 illustrating the forces exerted on a toner particle when the toner particle exits the drying chamber in accordance with the invention; and
FIG. 4. illustrates the steps of the invention.